Developer Frans Pop, author of debtree, posted an article showing the evolution in size of the GNOME desktop environment in recent Debian releases. The picture he paints isn't particularly pretty: the default GNOME install has increased drastically in size over the years.

But the problem is that Solid has been written from the ground up whose only consideration is Linux .

Wrong, it was writen as an abstraction layer above the OS, designed to handle different OS mechanisms. It abstracts away the OS spesific part for the application developers.

- if you want to make it available on a non-Linux system you're required to either change your operating system to be have more like Linux or you have to majorly change things in Solid.

No you don't, you only have to write Solid backends to talk to your OS spesific functions. Major changes to Solid is not needed, since this is exactly what it was designed to do. Think of it as Phonon for hardware. Besides Solid alrady have functionality for *BSD, Windows and OSX.

Add to the fact that Solid is a KDE project - I don't see it making its way over to GNOME anytime soon.

There you are right, the famed Gnome NIH syndrome will make that a certanty. More likely they will implement their own version in a couple of years.